Aqueous asbestos dispersion and process for producing same



Patented Dec. 1, 1953 UNITED STATES AQUEOUS ASBESTOS DISPERSION ANDPROCESS FOR PRODUCING SAME Glen D. Barbaras, Cleveland, Ohio, assignorto E. I. du Pont de Nemours & Company, Wilmington, Del., a corporationof Delaware No Drawing. Application November 15, 1949, Serial No.127,529

This invention relates to asbestos compositions and processes forproducing them and is more particularly directed to aqueous dispersionshaving a pH of from 3 to 6 and comprising up to about 5% by weight ofdiscrete, Crystallite, chrysotile asbestos fibers from 20 to 40millimicrons in diameter and, as a dispersing agent, a dissolved metalsalt having a monovalent anion and a cation containing a polyvalentmetal, the amount of dispersing agent being not more than enough to give3 milligram moles of dissociated anion per gram of asbestos, is furtherdirected to processes for producing such dispersions in which orientedchrysotile asbestos is disoriented by mechanical action in an aqueoussolution of the dispersing agent at a pH of from 3 to 6, and is stillfurther directed to dry products obtainable by drying the products ofsuch processes, said products consisting essentially of discreteCrystallite fibers of chrysotile asbestos about 20 to 40 millimicrons indiameter and, in intimate admixture therewith, a metal salt having amonovalent anion and a cation containing a polyvalent metal.

Chrysotile asbestos, as it occurs in nature, is made up of a mass oforiented fibers of a complex magnesium silicate. Masses of asbestos canquite readily be broken up by ordinary mechanical methods intorelatively coarse fibers which consist of bundles of fibrils. Byspecialized techniques such as those described in British Patent562,161, these relatively large fibers can be broken down into ultimatefibers estimated as having a diameter of the order of 300- Angstroms.However, in aqueous suspension as well as when dried, these ultimatefibrils are associated in discrete groups or fiocs easily observable bythe unaided eye, and because of this flocculation the system becomesnonhomogeneous and has properties characteristic of the fiocs.Therefore, it has not hitherto been possible to work with or onhomogeneously dispersed discrete individual fibrils hence theirusefulness has been greatly impaired.

Now according to the present invention it has been found that if thedisorientation of chrysotile asbestos by mechanical action is carriedout at a pH of from 3 to 6 in an aqueous solution containing as adispersing agent a metal salt having a monovalent anion and a cationcontaining a polyvalent metal, the amount of such metal salt being notmore than enough to give 3 milligram moles of dissociated anion per gramof asbestos, a homogeneous dispersion is obtained in which theindividual asbestos particles are not fiocculated. Such aqueousdispersions have a pH of from 3 to 6 and comprise up to about 5% byweight of discrete, C'rystallite, chrysotile asbestos fibers from 20 to40 millimicrons in diameter and, of

15 Claims.

course, the dispersing agent. They may be dried, the dry products,consisting essentially of the discrete Crysta1lite fibers and thedispersing agent, being readily redispersible in water to obtain thenon-flocculated dispersions.

The asbestos employed in the processes and products of this invention isknown as chrysotile. This is a fibrous form of asbestos and the mostcommon form. It may be mined from naturally occurring deposits andshould be cleaned of rocks and dirt before use. It preferably should bebroken up in a preliminary way, as in a chopping mill, such preliminarytreatment being of a character to break up very long fibers and fiberbundles which might otherwise interfere with the mechanical action meanssubsequently to be employed. Preliminary breaking up may be done in thedry way, as is the case in a chopping mill.

Following preliminary chopping the chrysotile is further broken down bymechanical action in aqueous dispersion. The oriented chrysotile becomesdisoriented into its ultimate, Crystallite fibrils by a combination ofmechanical action and chemical treatment under such conditions thatadverse chemical change of the fiber does not occur.

The mechanical action employed to effect disorientation must be of acharacter which tends to separate the individual fibrils from each otherbut which does not provide a high degree of attrition or grinding suchas would break the fibrils into very short pieces. A discussion ofvarious means applicable to this purpose is found in British Patent562,161 and any of the methods there disclosed for mechanical treatmentof the fibers in the presence of water may be used. As is there pointedout, the fibrous material is subjected to a bending and squeezing actionunder such conditions that the fibrous body has freedom to expand atsubstantially right angles to the direction of the applied force tospread out and liberate individual fibers as they are released from thepencil or bundle and that excessive breaking or cutting up of theindividual fibers is substantially avoided. A ball mill may be used forthis purpose and the mass thus produced may be broken up by stirring,vibration of the mass of liquid and asbestos, by spraying through anozzle or by blowing with air or steam or by explosive heat shatteringtreatment using steam, air or the like. Various types of beater millsmay be used and the mass may be subjected to ultrasonic treatment. Alaboratory mill of the Waring Blendor type, in which high shearingforces are produced when propeller-shaped knives are revolved at veryhigh speed, has been found to give excellent disorientation in a minimumof time with minimum reduction in fiber length.

According to the present invention it has been ound that mechanicalmeans alone are insuffi- :ient to give a deflocculated dispersion ofchrysoile. Rather, it has been found that control of he pH duringdispersion and also the presence if a dispersing agent is essential inorder to obain complete disorientation and avoid floccula- ;ion.

The dispersing agents in a process of this in- 'ention are dissolvedmetal salts having 2'. monivalent anion and a cation containing apolyalent metal. Such salts are not ordinarily bought of as beingdispersing agents. Alumiium chloride, for example, has been described nthe prior art as having the opposite effect on :hrysotile asbestos; (N.N. Serb-Serbina and i. Shumikhina J. Applied Chem. (U. S. S. R.) .6,273-9 (1943) However, under the conditions f the present invention thesesalts have been ound to have dispersing action on chrysotile. The saltsemployed must be sufficiently soluble n water to have the desiredaction, but it will be inderstood that even sparing solubility may be:ufiicient for this purpose. Thus, plumbous chlo- 'ide, PbClz, issufiiciently soluble to have a dis- Jersing action.

The anion of the dispersing agent must be nonovalent. It has been foundthat even traces )f polyvalent anions cause flocculation of the fi- )ersand their presence must be avoided. If pres- :nt as impurities in theraw materials they may )e removed by the addition of a small amount if aprecipitant, as for example, barium acetate remove sulfate ions. It isalso preferred not ;0 use the mcnovalent anion, fluoride, because ;hisanion has a pronounced tendency to form :omplex, polyvalent anions.Representative anons which may be used, in the order of theirefectiveness are the following radicals: formate, icetate, chloride,nitrate, bromide.

The polyvalent cation of the dispersing agent nay be a cation whichcontains only the polyralent metal or may be one which contains aiolyvalent metal in combination with another alement such as oxygen.Representative of the netals which may be present in the cation areiluminum, iron, chromium, zirconium, titanium, )eryllium, lead, copper,tin, indium, magnesium, )arium and zinc. Representative cations con-;aining another element in addition to the polyalent metal includezirconyl, titanyl and basic aluminum cations.

-It is particularly preferred to employ as a dis- )ersing agent a salthaving a cation containing tluminum, iron, chromium, zirconium or titan-.um, and of these the aluminum salt is outatanding.

The amount of dispersing agent used i criti- :a1. The minimum amount is,of course, that iecessary to effect dispersion. As little as 0.1nilligram molecular weights of a suitable salt ier gram of asbestos hasbeen found to give ef- :ective dispersion. The maximum amount ofdisiersing agent must be carefully controlled. The iermissable maximumis related to the character )f the anion formed when the metal salt isdis- :olved in water. When this anion does not atrongly associate withthe hydrogen ion present 11 the dispersion, the maximum amount ofdisaersing agent that can be added is less than when ;he anionassociates with the hydrogen ion to :orm a poorly dissociated acid. Forinstance, vhen a chloride is used, the chloride ion formed ipondissolution does not strongly associate ith iydrogen ion, whereas whenthe dispersing centration of anions in the dispersion may readily becalculated by reference to the dissociation 4 agent is an acetate, theacetate ion does associate with the hydrogen ion to form acetic acidwhich, being poorly ionized, ties up the acetate ion and reduces theeffective amount present. The maximum amount of any salt used must benot more than enough to give three milligram moles of dissociated anionper gram of asbestos. Since the anions of strong acids such ashydrochloric, hydrobromic, and nitric acid are, at this dilution,practically dissociated, it will be apparent that one milligrammolecular weight of a trivalent metal chloride such as aluminum chlorideor 1.5 milligram molecular weights of a chloride of a divalent metalsuch as beryllium is the maximum which can be tolerated. On the otherhand, when the acetate or formate of a polyvalent metal is used, theamount of dispersing agent can be larger because the anion'is.

poorly dissociated. The number of milligram moles of a salt which willgive a particular conconstant of the acid corresponding to the anion, atthe dilution and pH used.

In making a chrysotile dispersion by a proc-.

ess of this invention the pH of the aqueous me-. dium is maintained inthe range from 3 to 6. The ordinary pH of chrysotile dispersions is inthe range from 8 to 10, and until the pH i lowered to 6 no substantialamount of chemical dispersing action occurs. On the other hand if the pHis lowered below about 3, acid attack of the inpH of from 3 to 6. Theycomprise up to about" 5 per cent by Weight of chrysotile fibers, and thefibers are in the form of non-flocculated, discrete Crystallites from 20to 40 millimicrons in diameter. 7 The length of the fibers should beseveral times their diameter, and preferably as long as possible,provided adequate dispersion can be maintained. The dispersions alsocontain the dissolved dispersing agent as described above.

The novel, dry products of this invention are prepared by drying theaqueous dispersions. They consist of an intimate admixture of thediscrete Crystallite chrysotile fibers and the dis--- persing agent.They are readily redispersible in water to form the aqueous dispersionsabove described.

The aqueous asbestos dispersions of this invention are useful in themanufacture of formed asbestos products such as asbestos sheets. Thedried products provide a particularly useful form in which thedispersible material may be transported and stored with maximum economy.

A particularly useful type of sheet may be produced by filteringrelatively long fibers from their dispersions. Such dried sheets areunusual in their high tensile and tear strength, and flex re sistance.Their water resistance may be improved by treatment with reagentscommonly employed for such purpose, for example, alkyl siliconchlorides, stearato chromic chloride and the like.

This invention w'll be better understood by reference to the followingillustrative examples.

Example I opaque aqueous suspension of asbestos fibers was continuouslymixed for at least 3 minutes more while a one tenth molar solution ofone of the metal salts listed below was added dropwise until anexcellent dispersion of the fibers was obtained, and no markedimprovement resulted on further addition of the metal salt solution. Thedispersions rated excellent were thin, fiuid, translucent dispersions inwhich no agglomerates or fiber fiocs were visible to the unaided eye andin which individual fibers were not even visible under the crossedNicols of the light microscope. The electron microscope revealed thatthe fibers had diameters of to milli-microns and an average length inexcess of 30 microns, too long to be measured with the electronmicroscope.

The amounts of various salt solutions added to yield excellentdispersions are listed in the foly lowing table:

Crude chrysotile asbestos was chopped, mechanically fibrillated andtreated with dispersing agents in the Waring Blendor in a manner similarto that described in Example I. In this case, good dispersions wereobtained which were not quite as thin or translucent as the excellentdispersions obtained in Example I. The color of dispersion wascharacteristic of the metal salt used.

Cos. Mg. moles g ggggg MI 10 dispersmg dissociated Salt soln. agent perpH anion per per gm. gm. asgm asbestos bestos betos Ferric chloride 10 l3.1 3. 0 Chromic chloride l0 1 4. 9 3. 0 Beryllium acetate 10 l 5. o 1.7Titanyl chloride 1O 1 4.0 2.0 Magnesium acetate (15 cc. of M/lO aceticacid added to reduce pH)... 10 l 4. 8 1.9 Copper acetate l0 5. 7 1. 8

Example III Crude chrysotile asbestos was chopped, fibrillated andtreated with dispersing agents in the Waring Blendor as in Examples Iand II, but in this case, only a fair dispersion was obtained.

Cal

Ccs. Mg. moles 3 1 5332 M/lO dispersing dlss'oeiated Salt soln. agentper pH anion per per gm. gm. asgm asbestos bestos bes'ms Barium chloridel0 1 3.8 2.0 Stannic chloride l0 1 6 4.0 Indium nitrate 4 0.4 4. 5 1.2Stannous chloride 10 l 3 2.0 Aluminum bromide 8 0.8 4 2. 4

Example IV Chrysotile asbestos from Quebec, Canada, N0. 3 grade was notchopped but was directly fibrillated and treated with dispersing agentsin the Waring Blendor as in Examples I, II, and III with substantiallythe same results.

Example V Chrysotile asbestos from Quebec, Canada, No. 5 grade, obtainedalready milled, was fibrillated and treated with dispersing agents inthe Waring Blendor as in Examples I, III, and III with substantially thesame results but differing in that No. 5 grade fibers are shorter andcontain pulverized mineral impurities which readily settle out of thesuspensions and can be removed by decantation.

Example VI Crude chrysotile asbestos from Quebec, Canada, was handcobbed, picked free from adhering rock impurities and put through aWiley chopping mill to reduce the average fiber length to approximately4 inch. A suspension of 2.5 grams of this fiber in 250 cc. of distilledwater was charged into a one quart porcelain ball mill along with 1.5lbs. of glass beads T 5 inches in diameter. Two cubic centimeters of aone molar solution of aluminum chloride were added and the mixture wasmilled for 36 hours. An excellent thin translucent dispersion ofultimate asbestos fibrils resulted.

Example VII Five hundred grams of a 1% dispersion of chrysotile asbestoswas prepared according to the procedure of Example I and contained 0.15millimoles of aluminum acetate per gram of asbestos or a total of 0.153gram of aluminum acetate for the 5 grams of asbestos. The suspension wasdried at room temperature by evaporation. The dry product was broken upinto a fine voluminous fiufi in a hammer mill. Two grams of this productwere readily redispersed by mixing for two minutes in 198 grams of waterin the Waring Blendor.

Example VIII Three hundred grams of a A;% dispersion of #2 gradechysotile asbestos was prepared according to the procedure of Example IIand contained 0.28 millimole of aluminum acetate per gram of asbestos ora total of 0.0857 gram of aluminum acetate for 1.5 grams of asbestos.The dispersion was filtered through a finely porous ceramic block 8inches in diameter. The asbestos film remaining on the filter wasallowed to dry at room temperature. It was a soft, very flexible filmbut remarkably strong as indicated by a tensile strength of 700 lbs. persquare inch measured on the Scott Horizontal Tensile Tester and a tearstrength of 43 grams per cm. as measured by the .of discrete,Crystallite, chrysotile asbestos fibers rom 20 to 40 millimicrons indiameter and, as a.

lispersing agent, a dissolved metal salt having a nonovalent anion and acation containing a polyalent metal, the amount-oi dispersing agent bengenough to give from 0.1 milligram moles of he salt to 3 milligram molesof dissociated anion :er gram of asbestos.

2. An aqueous dispersion having a pH of .from

to 6 and comprising up to about 5% by weight f discrete, Crystallite,chrysotile asbestos fibers rom 20 to 40 millimicrons in diameter and adisolved aluminum acetate in an amount enough give from 0.1 milligrammoles of thesalt to 3 milligram moles of dissociated acetate ion per:ram of asbestos.

3. An aqueous dispersion having a pH of from to 6 and comprising up toabout 5% by weight f discrete, Crystallite, chrysotile asbestos fibersrom to millimicrons in diameter and a disolved aluminum formate in anamount enough 0 give from 0.1 milligram moles of the salt to 3 milligrammoles of dissociated format e ion per cram of asbestos.

4. An aqueous dispersion having a pH of from i to 6 and comprising up toabout 5% by weight if discrete, Crystallite, chrysotile asbestos fibersrom 20 to 40 millimicrons in diameter and a disolved zirconyl acetate inan amount enough to rive from 0.1 milligram moles of the salt to 3miligram moles of dissociated acetate ion per gram uf a sbestos.

- 5. In a process for producing an aqueous asrestos dispersion the stepcomprising disorienting Iriented chrysotile asbestos by agitation at apH f 3 to 6 in an aqueous solution containing as a :3.

6. In a process for producing an aqueous asestos dispersion the stepcomprising disorientng oriented chrysotile asbestos by agitation ata )Hof 3 to 6 in an aqueous solution containing A tn aluminum acetate in anamount enough to :ive from 0.1 milligram moles of the salt to 3 miligrammoles of dissociated acetate ion per gram )f asbestos.

' '7. In a process for producing an aqueous as- )BStOS dispersion thestep comprising disorienting )riented chrysotile asbestos by agitationat a pH if 3 to 6 in an aqueous solution containing an tluminum formatsin an amount enough to give 7 rom 0.1 milligram moles of the salt to 3.mill-igram moles of dissociated formate ion per gram )f asbestos. v

8. In a process for producing an aqueous as- :estos dispersion the stepcomprising disorienting ri'ented chrysotile asbestos by agitation at apH )f 3 to, '6 in an aqueous solution containing zir- :onyl acetate inan amount enough to give from L1 milligram moles ofthe salt to 3milligram noles of dissociated acetate ion per gram of as- Jest'os.

9. A dry composition consisting essentially of iiscrete Crystallitefibers of chrysotile asbestos ibout 20 to 40 millimicrons in diameterand, in ntimate admixture therewith, a metal salt havng a monovalentanion and cation containing a iolyvalent metal, the amount of said saltbeing :nough to give from 0.1 milligram moles of the ;alt to 3 milligrammoles of dissociated anion )BI gram of asbestos when the composition isdis- )ersed in water at a concentration of not more than 5% by weightof-asbestos and'the composition, when so dispersed, giving a pH 'of from3 tofi. V

10. A drycompositionconsisting essentially of discrete Crystallitefibers of chrysotile asbestos about 20 to 40 millimicrons in diameterand, in

intimate mixture therewith, an aluminum acetate in anamou'nt enough togive from 0.1 mil1i-= gram-moles of the salt to 3 milligram moles ofdissociated acetate ion per gram of asbestos when the composition isdispersed in water at a concentration of not more than 5 by weight oiasbestos.

11. A dry composition consisting essentially of discrete Crystallitefibers otchrysotile asbestos ab0'ut'20 to 40 millimicrons in diameterand, in'

intimate mixture therewith, an aluminum formate in an amount enough togive from 0.1 milligram moles of, the salt to 3 milligram. moles ofdissociated formate ion per gram of asbestos' intimate mixturetherewith, a zirconyl acetate in' an amount enough to give from 0.1milligram moles of the salt to 3 milligram moles of dissocie atedacetate ion per gram of "asbestos when the composition is dispersed inwater at a concentration of not more than 5% by weight of asbestos.

13. An aqueous dispersionhaving a pH of from 3 to 6 and comprising up toabout 5% by weight of discrete, Crystallite, chrysotile asbestos fibersfrom 20 to 40 millimicrons in diameter and, as a dispersing agent, adissolved metal salt selected from the group consisting of aluminumacetate,

aluminum formate, and zirconyl acetate, the' amount of dispersing agentbeing enough to give from 0.1 milligram moles of the salt to 3 milligrammoles of dissociated anion per gram of asbestos.

14. In a process for producing an aqueous asbestos dispersion the stepcomprising disorienting oriented chrysotile asbestos by agitation at apH of 3 to 6 in an aqueous solution containing as a dispersing agent ametal salt selected from the group consisting of aluminum acetate,aluminum formate, and zirconyl acetate, the amount of dispersing agentbeing enough to give from 0.1 milligram moles of the salt to 3 milligrammoles of dissociated anion per gram of asbestos.

15. A dry composition consisting essentially of discrete Crystalitefibers of chrysotile asbestos about 20 to 40 millimicrons in diameterand, in intimate admixture therewith, a metal salt selected from thegroup consisting of aluminum acetate, aluminum formate, and zirconylacetate, the amount of said salt being enough to give from 0.1 milligrammoles of the salt to 3 milligram moles of dissociated anion per gram ofasbestos when the composition is dispersed in water at 'a.

concentration of not more than 5% by weight of.

asbestos.

GLEN D. BARBARAS.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 1,907,616 Tucker May 9, 1933 FOREIGN PATENTS Number CountryDate 37,900 Germany Apr. 24, 1885 499,807 Great Britain Jan. 30, 1939

1. AN AQUEOUS DISPERSION HAVING A PH OF FROM 3 TO 6 AND COMPRISING UP TOABOUT 5% BY WEIGHT OF DISCRETE, CRYSTALLITE, CHRYSOTILE ASBESTOS FIBERSFROM 2/ TO 40 MILLIMICRONS IN DIAMETER AND, AS A DISPERSING AGENT, ADISSOLVED METAL SALT HAVING A MONOVALENT ANION AND A CATION CONTAINING APOLYVALENT METAL, THE AMOUNT OF DISPERSING AGENT BEING ENOUGH TO GIVEFROM 0.1 MILLIGRAM MOLES OF THE SALT TO 3 MILLIGRAM MOLES OF DISSOCIATEDANION PER GRAM OF ASBESTOS.